The measurement of the blood cell counts in a centrifuged sample of anticoagulated whole blood has been described in the scientific literature, and a method for easily measuring certain blood cell and other constituent layers is described in U.S. Pat. No. 4,027,660, granted Jun. 7, 1977 to Stephen C. Wardlaw et al. In the patented method, a sample of anticoagulated whole blood is centrifuged in a precision capillary tube which contains a plastic float. The float linearly expands some of the cell layers and the platelet layer.
In performing the patented method, the blood sample is centrifuged for about five minutes at about 12,000 RPM, and then the expanded lengths of the cell and platelet layers are measured. One of the problems in the patented method pertains to the need for the operator to remove the tube from the centrifuge and re-insert it into a reader. Because this operation must be performed within a limited time interval following centrifugation, it requires the close attendance of the technician, which is inefficient and further exposes the technician to a potentially hazardous sample. This problem is particularly important in that relatively unskilled technicians may not appreciate the need for timely reading of the sample tubes, and thus may result in erroneous readings. Another problem which arises with the aforesaid prior art method results from the fact that the capillary tube expands slightly as a result of the high pressure of the blood sample column during centrifugation of the blood sample. After the cell layers have been compacted around the float, they form a relatively solid plug in the bore of the tube and, as the centrifuge decelerates, the wall of the tube springs back to its normal shape, and thus tends to "pumps" the blood cell layers upwardly in the tube. This action causes disruption of at least a part of the cell layers in some samples and thus renders such cell layers difficult or impossible to read.
It would be desirable to be able to measure the blood constituent layers while the tube Is still under centrifugal stress so that the cell layers are not disrupted, and also to reduce the amount of sample tube handling and the potential for error.